While I disagree with the OP on smoking, I read the original question as: why vilify certain types of unhealthy behaviour as irresponsible (smoking), and not others (obesity)?
Now I appreciate that social pressure (aka vilification) has a role to play in getting us all to make healthier choices. The government is pouring lots of money into 'nudge' initiatives to get us to make healthier choices at the moment. It has the advantage of being cheap. But I worry that it's just a sticking plaster that avoids deeper changes that are needed.
For instance, I am not sure that vilifying people who make bad food choices is as effective as solving the underlying circumstances that are contributing to those choices. Like poverty (or, at the opposite end of the social spectrum, time poverty). Or the work done by the food industry to create magical thinking around food ('this chocolate bar can make you happy, sexy, bright-eyed and fulfilled').
To those who say 'it's up to the individual to be strong', I say this: whenever there is a public holiday, and especially at Christmas, I am always surprised at the number of people I see out jogging. The moment people have time to do so, many do start to look after themselves. But if we fill people's working lives with so much stress and pressure that they can barely get through the day, then their capacity to live healthy lives is reduced, and they are more likely to reach for high fat, high sugar, high salt foods rather than healthier options. So I would argue that we need structural change - like getting workplaces and working hours to be healthier and more family-friendly, has to be a huge part of the battle.